Building or central vacuum cleaner systems are known. These systems which are "built-in", i.e. fixed in a permanent position, have vacuum inlets which are connected by ducts to various parts of the building, providing vacuum through outlets in various parts of the building.
On/off switching of the vacuum cleaner power unit is controlled by completing and opening, respectively, an electrical connection at each vacuum outlet located throughout the building. Parallel electrical wires connect each vacuum outlet to the electrical control circuit that is part of the central vacuum cleaner power unit. Such switching at remote location, the location of each remote vacuum inlet, is described in U.S. Pat. No. 4,782,271 to Trowbridge.
The electrical switching control elements in vacuum cleaner power unit usually consists of a 24 VAC transformer and a single or multiple pole power relay.
When the electrical connection is completed at any of the vacuum inlets, or by an on/off switch located as part of the vacuum cleaner power unit, the alternating current from one leg of a 24 VAC transformer within the vacuum cleaner power unit flows through a relay energizing coil and completes connection through contacts at closed vacuum inlet switch to the other leg of the 24 VAC transformer. This alternating current flow energizes the relay coil closing the relay contacts and impressing 120/240 VAC from the power line voltage across one or more vacuum motors.
Therefore, before any means of electrical switch closure at the vacuum outlet, the voltage at the vacuum outlet is only 24 VAC, and the maximum current is limited by the relay coil resistance. This open circuit voltage of 24 VAC, provided by a class II approved transformer, has been certified safe by the Underwriter's Laboratory (UL) for direct human contact. The electrical connection points at the vacuum inlets are easily accessible to the touch of anyone operating the vacuum inlet, deliberately or accidentally, so safety is a primary requirement.
The weaknesses of prior art are the low reliability of high current relays, for motor control, and the unfavorable cost of relays and transformers compared to semiconductor devices.